In a captivating interview, Dr. David Deutsch, a foundational figure in the realm of quantum computing and proponent of the 'many worlds' interpretation of quantum theory, delves into the profound implications of quantum physics. He asserts that quantum theory is, without question, the corner-stone of modern physics, despite a tumultuous history of misinterpretation. Deutsch underscores a renaissance of understanding in the field, declaring the 'many worlds' interpretation as not only a plausible theory but the definitive framework through which to comprehend quantum phenomena.

With enthusiasm, he elaborates on the potential of quantum computers—machines that harness the power of quantum interference to perform computations far beyond the capabilities of traditional computers. Despite the excitement surrounding quantum technology, Deutsch acknowledges that we are still in the infancy of its development, with no universal quantum computer yet realized.

Deutsch's insights extend beyond the realm of computation; he boldly connects quantum theory to the metaphysical structure of the cosmos. He sees a vital interplay between the microcosm of quantum mechanics and the macrocosm of the universe itself. 'Quantum fluctuations,' he explains, can have far-reaching effects, influencing even the cosmic microwave background radiation—the afterglow from the Big Bang.

His engagement with quantum phenomena leads to fascinating discussions about parallel universes. Deutsch passionately argues that the equations governing quantum mechanics inherently suggest the existence of these many worlds. Although these universes typically operate independently, they can share insights during experiments, resulting in remarkable outcomes, including quantum interference and entanglement.

In a particularly mind-blowing revelation, he describes experiments utilizing photons to unveil the intricate interplay of quantum states across different universes, a testimony to the bold hypothesis of the many worlds theory. He articulates the unique functionality of quantum computers: instead of executing tasks through binary strings of 0s and 1s, they leverage the peculiar attributes of quantum particles, profoundly altering computational efficiency.

Deutsch, who first demonstrated that quantum computers could surpass the capabilities of Turing machines under certain conditions, recognizes the revolutionary potential of this technology. 'For some computations, the speed-up can be exponential—that's a game changer,' he states. Though he remains pragmatic, noting that no fully operational universal quantum computer exists at present, his optimism suggests that such developments are imminent on the horizon.

Another compelling aspect of the interview revolves around how these complex quantum interactions manifest in our comprehension of the universe. Deutsch humorously challenges the notion that human cognition is inadequate to grasp the profound nature of reality, countering with the assertion that we can indeed decode both the nano and macro worlds, albeit with some philosophical contortions—yes, spatial orientation varies by hemisphere!

Dr. Deutsch’s relentless drive to solve problems is refreshing; he emphasizes that productive inquiry isn't just about experimentation or theorization but involves a comprehensive approach to resolving foundational issues through creativity and discipline. 'All problems are local; solutions may be universal if fortune favors us,' he muses, flaunting his distinct blend of realism and attainable optimism.

He reveres Alan Turing’s ideas of computational universality, elaborating on how a single architecture can perform the same computations as any other entity. It’s clear that Deutsch bridges the thoughts of quantum mechanics and computing to a wider interdisciplinary landscape, advocating for a unified quest for knowledge.

As the conversation draws to a close, it's tangible that for Dr. Deutsch, the concept of the multiverse morphs into a symbol of the limitless possibilities that arise from quantum inquiries, computing advancements, and imaginative problem-solving techniques. A simple yet profound idea looms large: the adventure in science begins not with observations or theories, but with a lingering problem waiting to be addressed.

The horizon of quantum theory and its implications stretches far, hinting tantalizingly that perhaps some of those parallel universes already hold the answers we seek. Prepare to journey into the realm of quantum computing and cosmic wonder, for the answers lurking in the shadows of the multiverse may hold the key to understanding our very existence!